1. Field of the Invention
The present invention relates to composite-type cutting tools, whose drill portion and shank portion are made of different materials, and a method for producing such tools, for example, miniature drills used for making small-diameter holes in a printed circuit board.
2. Discussion of the Background
Generally, miniature drills are used for making holes having extremely small-diameters. Thus, miniature drills have: a cylindrical drill portion having a diameter of approximately 0.1 to 3.175 mm at the tip end of the drill body; and a larger-diameter shank portion at the rear end, which is used for holding the drill body in the axis of rotation of the machine tools. In general, the drill portion is formed from a cemented carbide.
Thus, to produce a solid-type miniature drill, the drill portion and the shank portion are integrally formed, for example, by grinding a cylindrical cemented carbide. Since such a cemented carbide is expensive and a large amount of the cemented carbide is ground for forming the drill portion, the cost inevitably increases.
Meanwhile, composite drills have been suggested which are produced as follows: a small-diameter drill portion is formed from a substantially cylindrical cemented carbide; a large-diameter shank portion is made from a low-cost material, such as steel or SUS, different from the material used for the drill portion- and the rear of the drill portion is allowed to fit into a hole made in the tip end of the shank portion.
FIGS. 12(A) to 12(E) show a method for producing such a conventional composite drill.
A shank portion 1 shown in FIG. 12(A) has a tapered portion 1a at the tip end and a hole 1b made in the end face of the tapered portion 1a.
According to shrinkage fitting, a substantially cylindrical drill portion 2 is pressed into the hole 1b while enlarging the diameter of the hole 1b by high-frequency heating (see FIG. 12(B)). Since the outer diameter of the drill portion 2 is set to be slightly larger than the inner diameter of the hole 1b, the inner wall of the hole 1b is shaved to produce chips by the insertion. The chips are pushed into the bottom of the hole 1b by the drill portion 2. The diameter of the hole 1b is then shrunk by cooling to achieve a tight fit.
The drill portion 2 can also be fitted to the hole 1b by brazing as follows: the drill portion 2 is pressed into the hole 1b having an inner diameter slightly larger than the outer diameter of the drill portion 2, and then, brazed.
Both sides of a joint 3, shown in FIG. 12(C), formed between the tapered portion 1a of the shank portion 1 and the drill portion 2 are finely ground to form a tapered face 3a having a smooth linear taper from the shank portion 1 to the drill 2 (see FIG. 12(D)), and the tip end of the drill portion 2 in succession to the tapered face 3a is ground to form a drill edge 2a (see FIGS. 12(D) and 12(E)).
However, since such methods for producing composite drills need a large number of steps, the process becomes complicated and laborious, disadvantageously increasing the cost. Furthermore, before pressing the drill portion 2 into the shank portion 1, the shank portion 1 is annealed by high frequency heating in the case of shrinkage fitting and by a flux in the case of brazing. Thus, the hardness of the shank portion 1 decreases so that the fastening strength due to the fitting to the drill portion 2 is disadvantageously reduced.
Furthermore, the drill portion 2 heated by high frequency heating is undesirably deformed when it is pressed into the hole 1b. The fastening strength due to fitting is also lowered by shaving the inner wall of the hole 1b. Furthermore, since the length of the drill portion 2 exposed outside the shank portion 1 varies with the amount of the shaved chips, it is necessary to adjust the length of the shank portion 1 to achieve a constant total length of the miniature drills.
Moreover, disadvantageously, the inner wall of the hole 1b is readily corroded by the flux at the time of brazing.
In addition, since the shank portion 1 and the drill portion 2 are made of different materials, it is difficult to simultaneously grind both of them into the linearly tapered face 3a over both sides of the joint 3 between the shank portion 1 and the drill portion 2.